Heat treatment of steel wires usually plays an important role in the art of wire-making. The first step in wire-making starts with drawing a wire rod to a desired intermediate diameter which can vary from 1.0 to 5.0 mm or more. At this stage of work-hardening the drawn wires are heat treated to pearlite by a patenting process to enable further plastic deformation. Subsequently, the patented steel wires are drawn to a smaller size, either a second intermediate size or a final diameter. Patenting involves heating carbon steel wires into the austenitic phase, generally above 800° C. and then cooling the wires to a chosen temperature held for a sufficient period for generally isothermal decomposition of the austenite to be completed. The temperature is usually in the region of 550° C., with the intention being generally to provide a fine pearlite structure.
As is well known, steel wire rod made by hot rolling from ingots or billets is applied to practical use in the rolled state but after having been subjected to a controlled cooling. In order to cool the high carbon rod immediately after having been hot-rolled to have an excellent cold-workability, it was proposed to dip the high carbon rod into a warm water bath as described in GB 1 276 738. The method of heat-treating a steel wire rod with a wire rod diameter ranging between 5.5 mm and 6.5 mm disclosed in this document comprises dipping the wire rod maintained at a temperature of from 600° C. to 1100° C. into a warm water bath containing a surface active agent. The water is held at a temperature higher than 45° C. thus generating a steam film uniformly on the wire rod surface and thereby controlling the cooling velocity of the wire rod. The essential point of this heat-treating method is to generate the steam film uniformly on the wire rod surface and to keep this state for some period of time until pearlite transformation has finished. Such a method has various merits when used in the direct cooling of hot rolled rods transported in spiral coils on a horizontal conveyor. However, this method has been regarded as being less suitable or unreliable for treatment of wires with other diameters.
Regarding the heat treatment of drawn wires having a desired intermediate diameter which can vary from 1.5 to 5.0 mm, EP 0 216 434 discloses another suitable and reliable method of controlled cooling of previously heated steel wire to an austenite temperature: the wire is transported continuously through a coolant bath containing substantially pure water of at least 80° C. and is immersed in the bath so as to effect a cooling to pearlite without producing martensite or bainite. The wire is subjected to uniform and stable film-boiled cooling along its entire immersion length by contacting the wire with a continuous non-turbulent flow of the substantially pure water. The water patented wires feature a sufficiently uniform pearlitic microstructure with excellent drawability records.
EP 0 524 689 also makes use of water of at least 80° C. as the coolant for the steel wire having a diameter which is less than 2.8 mm, but not continuously through a coolant bath as the aforementioned method disclosed in EP 0 216 434. Austenite to pearlite transformation may also be done in a water bath, however, if there is only one water bath provided, it may give problems for wire diameters smaller than 2.8 mm and even becomes impossible for wire diameters smaller than about 1.8 mm as the cooling velocity/speed of such a steel wire is too fast, which further causes unfavourable metallic structure of the patented steel wire. Thus, as an example desclosed in this EP patent, there are two water baths with air cooling in-between. The cooling is alternating done by film boiling in water during one or more water cooling periods and in air during one or more air cooling periods. A water cooling period immediately follows an air cooling period and vice versa, which is named as a “water-air-water patenting” process. The number of the water cooling periods, the number of the air cooling periods, the length of each water cooling period and the length of each air cooling period are so chosen so as to avoid the formation of martensite or bainite.
As explained in EP 0 524 689, all other technical parameters such as steel composition, coolant bath composition, temperature . . . being kept equal, the diameter of the wire plays a crucial part in the cooling speed. The smaller the diameter the greater the cooling speed, the greater the diameter the smaller the cooling speed.
WO2007/023696 relates to a direct heat treatment method of a loose coil-like rolled wire rod having a diameter more than 11.0 mm. The coil-like rolled wire rod are cooled by immersing them into refrigerant or exposing them to refrigerant flow.
Up to now prior attempts to use the aforementioned methods for the purpose of effecting a cooling-transformation of drawn and austenitized thick steel wires to pearlite, have been largely unsuccessful in many respects. The results of the heat treatment are too often unreliable and the treated thick wires show too high a variation in properties such as inconsistent drawability and frequently unexpected brittle behaviour because of numerous undesirable metallic structures. The exact metallic structure of the patented wire not only determines the absence or presence of wire fractures during the subsequent wire drawing but also determines to a large extent the mechanical properties of the wire at its final diameter. In this way, transformation conditions must be such that martensite or bainite are avoided even at very local spots on the steel wire surface. On the other hand, the metallic structure of the patented steel wire must not be too soft, i.e. it must not present too coarse a pearlite structure or too great a quantity of ferrite, since such a metallic structure would never yield the desired ultimate tensile strength of the steel wire. According to the statement of the previous paragraph, the essential point of carrying out a reliable thick wires' transformation-cooling is to accelerate cooling intentionally based on a conventional wire heat treatment process.